The present invention relates to transceivers. More specifically it relates to estimation of TX LO (transmitter local oscillator) leakage in a calibration for a transmitter utilizing direct conversion.
Wireless communication systems such as cellular phone networks, BLUETOOTH, GPS, and wireless local area networks (LAN) enable the transfer of data between devices in the system through the use of radio waves. Data transmitted from a source to a destination in a wireless system must typically be encoded into a manner suitable for transmission at a desired frequency. A transmitter, such as the one shown in FIG. 1A, formats an input signal (also referred to as a baseband signal) by modulating a carrier in the desired frequency band. The modulation may be implemented through amplitude, frequency, or phase variation of the carrier. An amplitude- and phase-modulated signal may be represented by means of in-phase and quadrature components, where the quadrature component is 90 degrees out-of-phase from the in-phase component. A receiver, such as the one shown in FIG. 1B, tuned to the same carrier frequency as the transmitter extracts the baseband signal from the modulated carrier through demodulation, which is the reverse process of modulation. A wireless device may have discrete transmitters and receivers, or it may have a transceiver, which integrates a transmitter and receiver into a single package.
Modulation poses significant implementation challenges. In a direct conversion transmitter, a local oscillator (LO) is used to upconvert a modulated analog baseband signal to a desired radio frequency. For example, in FIG. 1A, the TX (transmit) signal comprised of in-phase I(t) and quadrature Q(t) components is first filtered by baseband filters 110. Local oscillators 121, 122 generate a signal at a carrier frequency for converting I(t) and Q(t) to the carrier frequency using mixers 120 (also referred to as an upconverter). I(t) and Q(t) are then combined by summer 130, amplified by power amplifier 140, and transmitted as an RF signal.
LO leakage will typically occur when mixer 120 upconverts the baseband signal to a carrier frequency. LO leakage arises from DC offset in the TX baseband circuits as well as direct coupling of TX LO 121, 122 to the TX output. It degrades the transmission signal quality and creates in-band spurious emissions. Therefore, LO leakage should be measured and removed to improve signal quality and to conform to standards established by the FCC and other administrative bodies.
A TX LO leakage signal manifests as a DC signal in the receiver using the same LO frequency. Thus, TX LO leakage may be detected by measuring the DC offset in the receiver and cancelled by applying a DC offset corresponding to the level of LO leakage in the TX baseband. However, the measured DC offset in the receiver may be corrupted, thereby not accurately reflecting the TX LO leakage. Measurement is further complicated by a continuously running TX, wherein calibration must occur without taking the TX offline. Some factors that corrupt the RX (receiver) DC offset measurement are: LO leakage of the measurement circuitry itself, DC offsets in the baseband of the RX, phase differences between the received signal and the receiver's own LO signal, and LO harmonics arising from the use of non-linear mixers, such as switching mixers.
TX LO leakage measurement may be corrupted by the LO leakage of the measurement circuitry itself when the downconverter in the receiver uses the same LO signal as the TX. LO leakage of any components in the measurement circuitry will be downconverted to DC. Thus, the measured DC will not be purely due to TX LO leakage, but will also include any DC offsets from the measurement circuitry.
TX LO leakage measurement may be corrupted by phase difference between the received LO signal and the receiver-generated LO signal, when a signal is broken down and transmitted in its constituent in-phase and quadrature components. This is because the phase difference gives rise to coupling between the in-phase and quadrature components of the LO leakage signal. Additionally, phase may vary due to temperature and over time, and TX signaling frequency may vary as well.
TX LO leakage measurement may be corrupted if the mixers used for TX and RX are non-linear mixers, such as switching mixers. When a switching mixer is used for upconversion, the TX output will have a signal at the LO fundamental frequency corresponding to the TX LO leakage signal to be measured and removed, as well as additional signals at odd harmonics of the LO frequency. These LO harmonics at the TX output are downconverted by the switching mixer at the receiver due to conversion gains at all odd harmonics of the LO leakage signal, thereby corrupting the measurement of the TX LO leakage.
Therefore, there is a need in the art for accurately measuring the DC signal in the receiver corresponding to TX LO leakage, reducing or removing the error sources in the loopback circuitry, estimating the phase difference between the received carrier signal and the receiver LO signal, and reducing or removing LO harmonics when using a switching mixer.